Some hydration packs have very little space for mandatory kit.
I’ve used an OMM “Last Drop” 10 litre backpack, which I’ve found to be light and comfortable and doesn’t bounce around when running. There’s room in it for race kit (mobile phone, emergency blanket, head torch, packable jacket, etc) as well as emergency food (energy bars or gels). It is compatible with a hydration bladder if you want to use one, or couple of bottles will in the side pockets (in my case two 650ml bottles as there was a 1.5 litre requirement).
I’d recommend some sachets of energy drink or “zero” electrolyte tablets (easy to add to water in bottles) which also have the benefit of flavouring the water.
I use an OMM 'Last Drop' rucksack which is light and comfortable. It's fairly small at 10 litres, and currently on sale at Sweatshop. I'd second the advice to leave what you can at work, and minimise what you carry.